Refractive-diffractive spectrometer

ABSTRACT

A diffraction grating and a prism with the appropriate characteristics are employed to provide a combined dispersive characteristic that is substantially linear over the visible spectrum. Radiation from the grating and prism is collimated by a lens towards a detector array. The or a telecentric stop between the grating and prism is placed at a focal point of the lens in a telecentric arrangement so that equal magnification is achieved at the detector array. If the detector array is replaced by a plurality of optical channels, a multiplexer/demultiplexer is obtained.

BACKGROUND OF THE INVENTION

[0001] This invention relates in general to spectrometers and, inparticular, a refractive-diffractive spectrometer.

[0002] Spectrometers or spectrophotometers are widely used devices. Theyare used in digital printers and printing presses. Hand-heldspectrometers are used by graphic designers and imaging departments atnewspapers, magazines, and copy shops.

[0003] In order to analyze the spectro data from images or objects,light from the images or objects is passed through optical elements to adetector, such as a charged coupled device (“CCD”) array. In order toaccurately measure the CIE tristimulus values of light sources, imagesor objects, it would be desirable to accurately resolve all of thewavelength components in the radiation from the light source.Spectrometers resolve such wavelength components by dispersing them atdifferent angles depending on the wavelength. Unfortunately, up to thepresent time, spectrometers and spectrophotometers do not disperse thedifferent wavelengths linearly. This means that after being dispersed bythe spectrometer into the different wavelength components reaching theCCD array, the dispersion of a particular wavelength component is notproportional to the wavelength of the component. For example, if a prismis used in the spectrometer for dispersing the wavelength components ofradiation from a source, the angle of refraction of any wavelengthcomponent is not proportional to its wavelength.

[0004] Radiation from many light sources can have a large number ofspectral lines or wavelength components. Therefore, unless the CCD arrayhas the same number of detectors as the number of wavelength components,at least some of the spectral lines or wavelength components of thelight source will be directed to positions along the CCD array that doesnot fall entirely on any particular detector, but may fall partly on onedetector and partly on another detector. Since the dispersion of thewavelength components is nonlinear, it cannot be assumed that a linearinterpolation of the outputs of the two detectors will yield an accuratemeasurement of the intensities of such wavelength components. Thiscauses error in measurement. Therefore, to accurately measure the CIEtristimulus values of light sources of filters that have fine spectraldetail, spectrometers of the conventional design require higher spectralresolution. However, high quality spectrometers are expensive.

[0005] It is therefore desirable to provide improved spectrometers andspectrophotometers in which the above-described disadvantages areavoided.

SUMMARY OF THE INVENTION

[0006] This invention is based on the recognition that, by employing twooptical elements having a combined dispersive characteristic such thatthey substantially linearly disperse electromagnetic radiation over atleast a portion of an electromagnetic spectrum, the above-describeddisadvantages of conventional spectrometers and spectrophotometers canbe avoided. In the preferred embodiment, a refractive element such as aprism and a diffraction grating may be employed. Preferably, the twooptical elements are arranged so that the spectrometer is substantiallytelecentric; in such event, the spectrometer provides substantially thesame magnification at different wavelengths in the spectrum. In otherwords, radiation energy will be dispersed also linearly across theportion of the electromagnetic spectrum.

[0007] Where the combined dispersive characteristic of the two elementsis substantially linear, linear interpolation of the type describedabove would not introduce significant interpolation errors, in contrastto the conventional design of spectrometers. Therefore, even if highresolution CCD arrays are not used, radiation sources, images andobjects having a rich spectrum can still be accurately measured. Thisdrastically reduces the cost of the spectrometer.

[0008] Instead of actually detecting the different wavelengthcomponents, the wavelength components may be directed towards differentoptical channels in a demultiplexing arrangement. First, the light orradiation source may be an input optical channel carrying radiation ofdifferent wavelength components. After passing such wavelengthcomponents through the two optical elements, the wavelength componentsare dispersed substantially linearly. Therefore, irrespective of thewavelengths of the wavelength components in the input optical channel,one can be certain that a particular output channel is carrying acorresponding particular wavelength component. This is not possible ifthe combined dispersive characteristics of the two optical elements arenot substantially linear. The above demultiplexing arrangement isbidirectional. In other words, the separate output channels in the abovedemultiplexing arrangement can instead become input channels. Thewavelength components in such separate input channels, after passingthrough the two elements, will emerge as a combined beam towards theoutput channel (the input channel in the demultiplexing arrangement) ina multiplexer arrangement. Again, since the combined dispersivecharacteristic of the two elements is substantially linear, thedifferent wavelength components will be combined into a single beam bythe two elements irrespective of the wavelengths of the different inputwavelength components.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1A is a graphical plot of the natural spectral dispersion ofa diffraction grating useful for illustrating the invention.

[0010]FIG. 1B is a graphical plot of the natural dispersion of anoptical prism useful for illustrating the invention.

[0011]FIG. 1C is a graphical plot of the dispersion of the combinationof a diffraction grating and a prism.

[0012]FIG. 2A is a schematic view of the influx optics portion of aspectrometer to illustrate a preferred embodiment of the invention.

[0013]FIG. 2B is a schematic view of the spectrometer section of theembodiment of FIG. 3A.

[0014]FIG. 3A. is a schematic view of a diffraction grating, a prism anda lens to illustrate one combination of optical elements in thespectrometer to illustrate the invention.

[0015]FIG. 3B is a schematic view of a combined grating-prism-lensstructure in an embodiment to illustrate still another embodiment of theinvention.

[0016]FIG. 4 is a schematic view of a filter portion of the spectrometerto illustrate another aspect of the invention.

[0017]FIG. 5A is a schematic view illustrating the relative positions ofthe entrance slit image and of the exit slit or detector to illustrateconceptually the operation of the invention.

[0018]FIG. 5B is a graphical plot of the intercepted area of FIG. 5Aversus wavelength to illustrate the operation of the invention.

[0019]FIG. 6 is a graphical plot of the relative signal obtained byinterpolation to illustrate an aspect of the invention.

[0020]FIG. 7 is a graphical plot of the percent metameric error versusbandwidth to illustrate the invention.

[0021]FIG. 8 is a schematic view of a demultiplexer/multiplexer toillustrate another aspect of the invention.

[0022] For simplicity in description, identical components are labeledby the same numerals in this application.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0023]FIG. 1A is a graphical plot of the natural spectral dispersion ofa divergent light diffraction grating. As shown in FIG. 1A, adiffraction grating has higher dispersion of longer wavelengthcomponents compared to the shorter wavelength components in the visiblespectrum. Thus, as shown in FIG. 1A, wavelength components at or nearthe red end of the spectrum are dispersed by a greater amount comparedto wavelength components towards the blue end of the spectrum. FIG. 1Bis a graphical plot illustrating the natural dispersion of opticalprisms. As shown in FIG. 1B, the dispersion of prisms has the oppositecharacteristic compared to gratings. Prisms disperse wavelengthcomponents at or near the blue end of the visible spectrum by a greateramount compared to other wavelength components. Therefore, by employingboth a grating and a prism, the combined dispersive characteristic canbe made to be uniform over the entire visible spectrum, as illustratedin FIG. 1C.

[0024] The spectrometer of this invention is based on using refractionand a transmissive (or reflective) diffraction grating to produce anefficient and compact spectrometer. The design goal is a linearwavelength dispersion and constant spectral bandwidth. The goals areachieved by using a transmission grating in conjunction with a new formof a lens-prism. A grating, which has high dispersion in the red regionof the spectrum, is combined with a lens-prism that has high dispersionin blue spectral region results to produce constant spectral dispersionas shown in FIG. 1C in the same manner just as one would do toachromatize a multi-element lens. The slit magnification is maintainedby use of a telecentric stop in the lens-prism. The stop is placed at ornear the diffraction grating. The telecentric system maintains aconstant F/# across the spectrum and therefore constant slitmagnification. The telecentric system has the additional advantage thatthe detector array is illuminated normal to the array. Thisconfiguration minimizes cross talk between array elements.Alternatively, while the rays 30 may remain substantially normal to thedetector array, to avoid back-reflection, it may be desirable toslightly tilt the rays or the array so that the rays are at small anglesto the normal direction to the array.

[0025] With this brief description, the system and the design will beexplained in subgroups:

[0026] Influx Optics

[0027] The influx optics 10 comprises a field lens 12, a slit 14 and acollimator 16 as shown below in FIG. 2A. The field lens is placed justin front of the slit (slit can also be integrated with the lens) tocollect light from the target 18 and the slit is placed substantially ata focal point of collimating lens 16. The function of the field lens isto image the collimating lens to the sample area or target 18. With thisconfiguration, all of the light from the sample area 18 that passesthrough the spectrometer entrance slit also passes through thecollimating lens. This minimizes scattered light after the slit sinceall light leaving the slit will be directed toward the collimating lens.

[0028] The width of slit 14 defines the spectral resolution of thesystem. The slit height is chosen to fill the height of the elements ofthe detector array in the spectrometer section 20 of FIG. 2B. For agiven spectral resolution, the slit width is the product of the focallength of the collimator and angular resolution of the grating of FIG.2B. Once the spectral resolution and the target area are chosen thecollimator dimensions are also defined. In one embodiment, a large slitand a long F/10.0 collimator are used. This makes both the slit and thelens easy to manufacture.

[0029] The collimating lens and the following stops define the field ofview of the instrument at the target plane. The entrance slit 14 isplaced substantially at a focal point of collimating lens 16 so that thecollimating lens images the entrance slit to infinity so that the lightfalling on the grating is collimated (all light rays from a point areparallel). The grating could be illuminated with diverge light.Unfortunately, divergent light passing through the gating producescomatic aberration. The extra aberration would increase the bandwidth ofthe instrument as a function of wavelength.

[0030] The Spectrometer

[0031] The spectrometer section 20 has four parts: a grating 22, atelecentric stop 24, a lens-prism 26 and detector array 28, such as aCCD array. These elements are shown in FIG. 2B. The collimated lightfrom the front section 10 passes through and diffracted, or is reflectedand diffracted, by the grating and is dispersed into its spectralcomponents as shown in FIG. 2B. These rays are further dispersed by thefirst surface of the lens-prism element. In one embodiment, the gratingis ruled at 600 lines/mm. The grating frequency and the prism angle arepart of the design optimization. The lens-prism comprises a prismportion 26 a and a lens portion 26 b. The last variable in theoptimization is the choice of which wavelength will strike normal to thesecond face 26′ of the prism portion. It is this ray (the center ray 30′in FIG. 2). that is the optical axis of lens portion 26 b that makes upthe second face of the lens-prism. The grating frequency, prism angleand wavelength of the chief ray (optical axis) are chosen to produce asystem with linear spectral dispersion. Any one of the commonly usedoptical component design programs may be used to provide a preciseoptical design for a system with linear spectral dispersion given theabove inputs (grating frequency etc.) so that it is not necessary todescribe the process in detail.

[0032] In one embodiment, the grating frequency is targeted to be in therange of 500 to 600 lines/mm. The frequency is kept low to minimizepolarization efficiency differences. At higher frequencies polarizationscramblers would be required to compensate for these polarizationdifferences. The grating is blazed for the first order. The blaze angleis determined by the spectral sensitivity of the system. The wavelengthof the maximum efficiency is placed where the system has the leastsensitivity. In general the optimum solution is to have the same signalstrength from each portion of the spectrum when reading a perfect whitereference.

[0033] The lens-prism 26 is designed so that the first or higher orderdiffracted light is collimated and transmitted by the grating 22 andfocused on the detector array 28. The length of the prism is determinedby the focal length of the final lens surface of lens portion 26 b. Theface 26″ of the prism portion next to the grating is approximately atthe infinity focus of the Lens-Prism. As shown in FIG. 2B, rays 30 oflight leaving a point near the grating leave the final lens in acollimated state (parallel). This is the telecentric condition. Toachieve such condition, the telecentric stop 24 is located at orsubstantially at a focal point of the lens portion 26 b. If no stop 24is employed, the grating 22 is located at or substantially at a focalpoint of the lens portion 26 b. This enables substantially equalmagnification of all wavelength components in all the rays 30. Surface26″ may be aspheric as shown in dotted line in FIG. 2B to compensate forgeometric aberration introduced by the collimating lens 16 of FIG. 2A.

[0034] The detector array 28 is positioned to be in focus at eachwavelength and substantially perpendicular to the telecentric raysleaving the final lens surface of lens portion 26 b. This position ofthe array 28 minimizes cross talk between elements and assures constantslit magnification and therefore constant spectral bandwidth at everydetector site in the array.

[0035] While in the preferred embodiment, a lens-prism 26 is employed,it is also possible to use a separate prism and a separate lens instead,as illustrated in FIG. 3A. In such event, the telecentric stop 24 isplaced at a focal point of lens 34. Where no stop 24 is used, grating 22is placed at a focal point of lens 34. In the same manner as illustratedabove in reference to FIG. 2A, 2B, grating 22 of FIG. 3A may act as atransmissive or reflective grating. In other words, light (preferablycollimated) from the target may pass through and diffracted by grating22 and proceed towards prism 32 and lens 34, or is reflected anddiffracted by the grating. In any event, the first or higher orderdiffraction passes through prism 32 and lens 34 and is incident on adetector array such as array 28 (not shown). In still anotherarrangement, all three elements: the grating, the prism and the lens maybe combined into a single optical element 40 as shown in FIG. 3B. Insuch event, the grating surface 40′ is located substantially at a focalpoint of the lens portion 40″ of the element.

[0036]FIG. 4 is a schematic view illustrating a filter placed in frontof the detector array 28 to further reduce cross-talk in thespectrometer of this invention. As shown in FIG. 4, the rays (such asrays 30 of FIG. 2B) that are directed towards the detector array arefirst passed through corresponding filter elements 50 before thefiltered rays are then directed towards the detector array 28. Whileonly seven filters 50 are shown in FIG. 4, comprising blue, blue-green,green, yellow, orange, red-orange and red filters, it will be understoodthat more or fewer filters may be employed, which is within the scope ofthe invention.

[0037] Explained below is the reason why the new Grating-Prism-Lens(GPL) Spectrometer of this invention has the colorimetric accuracy of aspectrometer with a much smaller bandwidth. A 1 mn bandwidthconventional system is compared to a 10 nm GPL system.

[0038] The GPL spectrometer (such as system 20 of FIG. 2B) has beendesigned to have two unique features. The first is that by using a prismcombined with a grating, the dispersion of the system is made linear orsubstantially linear over the visual wavelength region of 400 nm to 700nm. By using a prism combined with a grating, the dispersion of thesystem can also be made linear or substantially linear over a portion ofthe spectrum that includes wavelengths in the infrared or ultravioletrange. Thus, the spacing between 10 nm samples occurs at equal distancesin the image plane. Second, the use of the telecentric conditionproduces a system that has constant magnification of the slit for allwavelengths. Therefore, an array detector with 10 nm spacing betweendetectors will be just filled with light by a line source that is at thecenter wavelength of the detector.

[0039]FIG. 5A illustrates how the image of the entrance slits, fromwavelengths differing from the center wavelength of the detectorelement, move across the array element. The detector element acts as theexit slit of the system. FIG. 5A shows how much of the light isintercepted for each of the line spectra. The images 14′ of the slit 14of FIG. 2B on the exit slit (detector) are shown for wavelengthdifferences of −6 nm, −3 nm, 0.0 nm, 3 nm and 6 nm. The amount of energyintercepted by the exit slit is directly proportional to the differencebetween the center wavelength of the exit slit position and thewavelength of the entering light. Therefore, if the wavelengthdifference is zero, all the light from the entrance slit will fall onthe exit slit. As is shown in the plot on FIG. 5B, the amount of lightintercepted by the exit slit is a linear function of the differencebetween wavelengths. The amount of light intercepted by the exit slit iszero for differences of 10 nm or greater. The amount of light falling ona detector is a triangular function of the difference in wavelengthbetween the center wavelength and the wavelength of the entering light.

[0040] The GPL is different from other spectrometer configurations. Allothers have some form of uncorrected dispersion. As such, none of themexhibit the natural interpolation abilities of the GPL spectrometer. Allother spectrometers require higher spectral resolution to accuratelymeasure the CIE tristimulus values of source or filters that have finespectral detail. The GPL spectrometer can determine the CIE tristimulusvalues of line spectra without the need to resolve the line spectra. Thenext section illustrates the ability of the GPL system.

[0041] GPL CIE Tristimulus Determination

[0042] Spectrometers in use today require a resolution of 1 nm todetermine the tristimulus values of sources that have line spectraincluded in a continuum spectrum. Fluorescent lamps and monitorphosphors are good examples of problem sources that normally requirehigh spectral resolution. The GPL spectrometer has the ability ofobtaining accurate CIE tristimulus values without the need to resolvethe spectral lines. Since all radiation is just the sum of an infinitenumber of spectral line sources, the GPL will yield accurate tristimulusvalues for all light distributions. The following discourse willillustrate the ability of the GPL system to produce exact CIEtristimulus values for an infinitesimal width spectral line source.

[0043]FIG. 6 shows the spectral response functions of two adjacentdetectors in an array of detectors. An arbitrary and perfect line sourceis received by the GPL system that has a wavelength that is betweenthese detectors. The source wavelength is assumed to be placed at adistance a times the spectral distance between the adjacent detectors.

[0044] A perfect spectrometer with a small enough bandwidth is assumedto resolve the source given above. One would then know the exact centerwavelength of the source and the energy of the source, Es. Thetristimulus value of the source would be computed by linearinterpolation. The known source wavelength lies between two wavelengthswhere the CIE color mixing functions have been tabulated. Thetristimulus value is given by linear interpolation:

T=M1*Es*(1-α)+M2*Es*α  (1)

[0045] where M1 is the color mixing function at the lower wavelength andM2 at the upper wavelength. The source wavelength lays a proportionatedistance α from the lower wavelength.

[0046]FIG. 6 shows how the source is seen by the GPL spectrometer. Theoriginal energy passing through the entrance slit falls on bothdetectors. Detector 1 receives E1 amount of energy and detector 2receives E2. The tristimulus value would be calculated as follows:

T=W1*E1+W2*E2   (2)

[0047] From FIG. 6, one can see that the triangular band pass of GPLdevise splits the energy, Es, as follows:

E1=Es*(1-α)   (3)

and

E2=Es*α  (4)

[0048] Now if equations 3 and 4 are substituted for E1 and E2 inequation 2, we obtain:

T=W1*Es*(1-α)+W2*Es*α  (5)

[0049] This is the same result obtained by the spectrometer withinfinite resolution. Therefore, if the use of the spectrometer is todetermine the tristimulus values of a source, the GPL spectrometer willgive the proper result. If the purpose is to accurately resolve thespectral components of the source, then the broad band GPL is not theappropriate tool.

[0050] Bandwidth Considerations

[0051] The final question in the design of the GPL device is thewavelength spacing of the samples. The choice was made to use 10 nmspacing. This choice is a compromise between accuracy and sensitivity.The sensitivity of the instrument increases with increase of bandwidth.Unfortunately the metamerism error in the computation of tristimulusvalues increases with spectral bandwidth. Triangular filtering a 1 nmtable of color mixing functions to a 3, 10 and 20 nm bandwidth simulatedthe optical actions of each spectrometer. The metameric error iscalculated by subtracting the linear interpolated data from the true 1nm color mixing functions. FIG. 7 displays the increase of standarddeviation of metameric error as a function of spectral bandwidth. Aspectral resolution of 10 nm is chosen as optimum compromise betweensensitivity and metameric error. Note that the error difference betweena 3 nm and a 10 nm bandwidth is small as compared with an instrumentwith a 20 nm bandwidth.

[0052] The GPL spectrometer has been designed to have linear dispersionand constant slit magnification over the spectral region of 400 to 700nm. As a result the instrument can measure the CIE Tristimulus values ofsources that contain very spiky spectral components. An analysis is madeof the metameric error in the weighting functions as a result of usingthe triangular spectral band pass as an interpolator. It is found that10 nm was the upper limit of the allowed spectral band pass to limitmetameric error to a value of 0.001639 RMS of the true value of thecolor mixing functions. This value is only slightly larger that a systemwith a 3 nm bandwidth.

[0053] Multiplexer/Demultiplexer

[0054] The above-described diffraction grating and prism may also beused in optical multiplexers and demultiplexers as illustrated in FIG.8. Thus, in reference to FIGS. 2A and 2B, if the radiation or lightcollected by field lens 12 originates from an optical channel such as anoptical fiber, such radiation is collected by lens 12 in the same manneras that described above. Radiation from collimator 16 then undergoesdiffraction and other optical operations in FIG. 2B, except that insteadof being detected by a detector array 28, each of the rays 30 iscollected by a corresponding output optical channel. In other words, thedifferent wavelength components from an input optical channel isdispersed into its different wavelength components where each wavelengthcomponent is then collected by a corresponding output optical channel,thereby functioning as an optical demultiplexer.

[0055] Thus, as shown in FIG. 8, each of the three arrows 102 is aninput optical channel. For each input optical channel 102, there is acorresponding optical system comprising lens 12, slit 14, collimator 16,grating 22, stop 24, lens-prism 26 and the separated wavelengthcomponents are collected by a plurality of three output opticalchannels. While only three output optical channels 104 are showncollecting the wavelength components originating from each input opticalchannel 102, fewer or more output optical channels may be employed andis within the scope of the invention. Thus, output optical channels 104′are used to collect rays 30 that originate from input optical channel102′, and direct these optical signals to corresponding receivers (notshown). The same advantages of linear dispersion and equal magnificationcan advantageously be used in the demultiplexer 100. Since thedispersion is linear, it would be easy to identify the spacing andlocation of the output optical channels 104, if the expected wavelengthsof the input radiation is known. It would then be much simpler to designthe demultiplexer 100. Since system 100 provides substantially the samemagnification for all wavelength components, there would be little or nodistortion or unequal reduction of intensity of the wavelengthcomponents between the different wavelength components in the differentoutput channels.

[0056] As shown in FIG. 8, where the input and output optical channelsare optical fibers, a GRIN lens (not shown) may be used as an opticalinterface for each of the optical channels in FIG. 8.

[0057] The passage of radiation in system 100 is bidirectional. Thismeans that system 100 may also function as a multiplexer by convertingeach of the input optical channels 102 into output optical channels andthe output optical channels 104 into input optical channels. In suchevent, the different wavelength components carried by the input opticalchannels 104′ would then be combined and directed towards the outputoptical channel 102′ in such multiplexer. Again, if the optical channelsare optical fibers, GRIN lenses may be employed as optical interfaces.

[0058] While the invention has been described above by reference tovarious embodiments, it will be understood that changes andmodifications may be made without departing from the scope of theinvention, which is to be defined only by the appended claims and theirequivalent. For example, while it is preferred to employ a refractiveelement and a grating so that their combined dispersion is substantiallylinear, it may be adequate for some applications where the combineddispersion over at least a portion of an electromagnetic spectrum is notlinear, but nevertheless more even than by only one of the two elements.All references referred to herein are incorporated by reference in theirentireties.

What is claimed is:
 1. An apparatus, comprising at least a first and asecond element that disperse electromagnetic radiation according towavelength, said at least two elements having a combined dispersivecharacteristic such that they substantially linearly disperse theelectromagnetic radiation over at least a portion of an electromagneticspectrum.
 2. The apparatus of claim 1, said at least two elementsincluding a refractive element and a grating.
 3. The apparatus of claim2, wherein said at least two elements include a prism.
 4. The apparatusof claim 1, wherein said portion of the spectrum includes wavelengths inthe infrared or ultraviolet range.
 5. The apparatus of claim 1, whereinsaid apparatus provides substantially the same magnification of theentrance slit at the detector array at different wavelengths in thespectrum.
 6. The apparatus of claim 5, wherein said at least twoelements are arranged so that the apparatus is substantiallytelecentric.
 7. The apparatus of claim 6, further comprising a lens,said at least two elements including a grating placed substantially at afocal point of the lens that focuses the electromagnetic radiation fromthe grating to a detector array.
 8. The apparatus of claim 6, said atleast two elements comprising a lens component, and a grating placedsubstantially at a focal point of the lens that focuses theelectromagnetic radiation from the grating to a detector.
 9. Theapparatus of claim 8, said at least two elements comprising a prismintegral with the lens component.
 10. The apparatus of claim 6, furthercomprising a lens and an aperture stop, said at least two elementscomprising a grating, said aperture stop being located in an opticalpath between the grating and the lens, said aperture stop placedsubstantially at a focal point of the lens that focuses theelectromagnetic radiation from the aperture stop to a detector.
 11. Theapparatus of claim 6, further comprising an aperture stop, said at leasttwo elements comprising a grating and a lens component, said aperturestop being located in an optical path between the grating and the lens,said aperture stop placed substantially at a focal point of the lensthat focuses the electromagnetic radiation from the aperture stop to adetector.
 12. The apparatus of claim 1, further comprising a lens, saidat least two elements including a prism and a grating, wherein theprism, lens and grating are separate components.
 13. The apparatus ofclaim 1, further comprising a lens, said at least two elements includinga prism and a grating, wherein any two or more of the prism, lens andgrating are combined to form a unitary single optical component.
 14. Theapparatus of claim 13, wherein the prism, lens and grating are combinedto form a unitary single optical component.
 15. The apparatus of claim1, said at least two elements including a prism, a grating and a lenscomponent integral with the prism.
 16. The apparatus of claim 1, said atleast two elements including a transmissive or reflective grating. 17.The apparatus of claim 1, further comprising an array of detectors, eachdetector sensitive for detecting a range of wavelengths ofelectromagnetic radiation in the spectrum from the at least twoelements, and a filter corresponding to each detector in the array, suchfilter filtering out at least some of the wavelengths of electromagneticradiation that are not within the range of wavelengths ofelectromagnetic radiation of the corresponding detector.
 18. Theapparatus of claim 1, further comprising an array of detectors, whereinthe electromagnetic radiation at different wavelengths arrive at thearray in substantially parallel rays.
 19. The apparatus of claim 18,said array of detectors being substantially in a plane, wherein the raysof the electromagnetic radiation at different wavelengths arrive at thearray in directions that are substantially normal to the plane.
 20. Theapparatus of claim 19, wherein directions of the rays of theelectromagnetic radiation at different wavelengths arriving at the arrayare at small angles to a normal direction to the plane to avoid backreflection.
 21. The apparatus of claim 1, said at least two elementscomprising a prism having a surface receiving the electromagneticradiation from a collimating lens, said surface being aspheric tocompensate for geometric aberration introduced by the collimating lens.22. An optical apparatus, comprising: at least a first and a secondelement that disperse electromagnetic radiation according to wavelength,said at least two elements having a combined dispersive characteristicsuch that they substantially linearly disperse the electromagneticradiation over at least a portion of an electromagnetic spectrum; andinput/output optical channels conveying electromagnetic radiationsignals to or from the at least two elements, said at least two elementsmultiplexing or demultiplexing the electromagnetic radiation signals.23. The apparatus of claim 22, said at least two elements including arefractive element and a grating.
 24. The apparatus of claim 23, whereinsaid at least two elements include a prism.
 25. The apparatus of claim22, wherein said portion of the spectrum includes wavelengths in theinfrared or ultraviolet range.
 26. The apparatus of claim 22, whereinsaid apparatus provides substantially the same relative magnification ofan entrance slit and an image thereof at the channels at differentwavelengths in the spectrum.
 27. The apparatus of claim 26, wherein saidat least two elements are arranged so that the apparatus issubstantially telecentric.
 28. The apparatus of claim 27, furthercomprising a lens, said at least two elements including a grating placedsubstantially at a focal point of the lens that focuses theelectromagnetic radiation from the grating to channels, or from thechannels to the grating.
 29. The apparatus of claim 27, said at leasttwo elements comprising a lens component, and a grating placedsubstantially at a focal point of the lens that focuses theelectromagnetic radiation from the grating to the channels, or from thechannels to the grating.
 30. The apparatus of claim 29, said at leasttwo elements comprising a prism integral with the lens component. 31.The apparatus of claim 27, further comprising a lens and an aperturestop, said at least two elements comprising a grating, said aperturestop being located in an optical path between the grating and the lens,said aperture stop placed substantially at a focal point of the lensthat focuses the electromagnetic radiation from the aperture stop to thechannels, or from the channels to the stop.
 32. The apparatus of claim27, further comprising an aperture stop, said at least two elementscomprising a grating and a lens component, said aperture stop beinglocated in an optical path between the grating and the lens, saidaperture stop placed substantially at a focal point of the lens thatfocuses the electromagnetic radiation from the aperture stop to thechannels, or from the channels to the stop.
 33. The apparatus of claim22, further comprising a lens, said at least two elements including aprism and a grating, wherein the prism, lens and grating are separatecomponents.
 34. The apparatus of claim 22, further comprising a lens,said at least two elements including a prism and a grating, wherein anytwo or more of the prism, lens and grating are combined to form aunitary single optical component.
 35. The apparatus of claim 34, whereinthe prism, lens and grating are combined to form a unitary singleoptical component.
 36. The apparatus of claim 22, said at least twoelements including a prism, a grating and a lens component integral withthe prism.
 37. The apparatus of claim 22, said at least two elementsincluding a transmissive or reflective grating.
 38. The apparatus ofclaim 22, each channel carrying a wavelength component within a range ofwavelengths of electromagnetic radiation in the spectrum from the atleast two elements, and a filter corresponding to each channel, suchfilter filtering out at least some of the wavelengths of electromagneticradiation that are not within the range of wavelengths ofelectromagnetic radiation of the corresponding channel.
 39. Theapparatus of claim 22, wherein the electromagnetic radiation atdifferent wavelengths arrive at the channels in substantially parallelrays.
 40. The apparatus of claim 39, said channels have endssubstantially in a plane, wherein the rays of the electromagneticradiation at different wavelengths arrive at or emerge from the array indirections that are substantially normal to the plane.
 41. The apparatusof claim 40, wherein directions of the rays of the electromagneticradiation at different wavelengths arriving at the array are at smallangles to a normal direction to the plane to avoid back reflection. 42.The apparatus of claim 41, said at least two elements comprising a prismhaving a surface receiving the electromagnetic radiation from acollimating lens, said surface being aspheric to compensate forgeometric aberration introduced by the collimating lens.
 43. Anapparatus, comprising at least a first and a second element thatdisperse electromagnetic radiation according to wavelength, said atleast two elements having a combined dispersive characteristic such thatthey disperse the electromagnetic radiation over at least a portion ofan electromagnetic spectrum more evenly than by only one of the twoelements.
 44. A method for measuring a radiation source, comprising:passing radiation from the source to at least a first and a secondelement that disperse electromagnetic radiation according to wavelength,said at least two elements having a combined dispersive characteristicsuch that they substantially linearly disperse the electromagneticradiation over at least a portion of an electromagnetic spectrum; andmeasuring wavelength components of the radiation that is dispersed bythe elements.
 45. A optical method for multiplexing or demultiplexingelectromagnetic radiation signals, comprising: providing at least afirst and a second element that disperse electromagnetic radiationaccording to wavelength, said at least two elements having a combineddispersive characteristic such that they substantially linearly dispersethe electromagnetic radiation over at least a portion of anelectromagnetic spectrum; and conveying electromagnetic radiationsignals to or from the at least two elements, said at least two elementsmultiplexing or demultiplexing the electromagnetic radiation signals.